1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device for providing reliable image quality.
2. Discussion of the Related Art
Today's information age demands various forms and types of display devices. In order to satisfy such an increasing demand, there is much ongoing research for developing flat panel display devices such as a liquid crystal display device (LCD), a plasma display panel (PDP) and an electro luminescent display (ELD). Some flat panel display devices have already been utilized in various types of equipment.
Among flat panel display devices, liquid crystal display devices (LCDs) are widely used as a mobile display device instead of using a cathode ray tube (CRT) display device because LCDs are lightweight, slim, and do not consume much power. LCDs are used as monitors for notebook computers and for televisions, for example.
Liquid crystal display devices display images by affecting light transmissivity by controlling the arrangement of liquid crystal molecules.
FIG. 1 is a schematic block diagram of a related art liquid crystal display device.
As shown in FIG. 1, the related art liquid crystal display device includes a liquid crystal panel 2 where a predetermined image is displayed, a PCB 4 disposed at one side of the liquid crystal panel 2, a plurality of data TCPs 10a to 10c interposed between the one side of the liquid crystal panel 2 and the PCB 4, a plurality of data driver ICs 12a to 12c embedded in corresponding data TCPs 10a through 10c, a plurality of gate TCPs 14a and 14b disposed at another side of the liquid crystal panel 2, and a plurality of gate driver ICs 16a and 16b embedded in corresponding gate TCPs 14a and 14b. 
The PCB 4 includes various elements. For example, the PCB 4 may include a timing controller 6, a common voltage generator 8 and a power supply (not shown). The timing controller 6 generates a gate control signal for driving the gate driver ICs 16a and 16b and a data control signal for driving the data driver ICs 12a through 12c. The gate control signal is transmitted to the gate driver ICs 16a and 16b through a predetermined gate signal line (not shown). The data control signal is transmitted to the data driver ICs 12a through 12c through a predetermined data signal line (not shown). The common voltage generator 8 generates a predetermined level of a common voltage.
The liquid crystal panel 2 may include a lower substrate 1, an upper substrate 3 and liquid crystal interposed between the lower substrate 1 and the upper substrate 3. A plurality of gate lines GL1 to GLn and a plurality of data lines DL1 to DLm are formed on the lower substrate 1 and the gate and data lines intersect each other, and a thin film transistor (TFT) and a pixel electrode (not shown) are formed at the crossing of the gate lines GL1 to GLn and the data lines DL1 to DLm. Also, a plurality of common lines VL1 to VLn is formed to be parallel to a plurality of gate lines GL1 to GLn. The pixel electrode forms a storage capacitor Cst by overlapping with the common lines. The related art liquid crystal display device is called a storage on common mode. R, G and B color filters are arranged on the upper substrate 3 and a common electrode is formed on the R, G, B color filters.
The common voltage Vcom generated at the common voltage generator 8 is supplied to a first TCP line 17a and a second TCP line 17b. The first TCP line 17a is formed on the first data TCP 10a in a patterned shape and the second TCP line 17b is formed on the third data TCP 10c in a patterned shape.
A first to a fifth common voltage supplying line 18a to 18e are arranged at edge regions of the lower substrate 1. The first and the second common voltage supplying line 18a and 18b are arranged at a left edge region of the lower substrate 1, and the third and the fourth common voltage supplying line 18c and 18d are arranged at a right edge region of the lower substrate 1. The fifth common voltage supplying line 18e is arranged at a bottom edge region of the lower substrate 1. Furthermore, a silver (Ag) dot 22 is formed roughly at an edge region of the lower substrate 1 for transmitting a common voltage between the lower substrate 1 and the upper substrate 2.
The first TCP line 17a is commonly connected to the first and the second common voltage supplying lines 18a and 18b, and the second TCP line 17b is commonly connected to the third and the fourth common voltage supplying lines 18c and 18d. 
The first, the third and the fifth common voltage supplying lines 18a, 18c and 18e are electrically connected to the Ag dot 22. That is, the first and the third common voltage supplying lines 18a and 18c pass a common voltage to a common electrode of the upper substrate 3 through the Ag dot 22. The second and the fourth common voltage supplying lines 18b and 18d are electrically connected to the common lines VL1 to VLn. Therefore, the second and the fourth common voltage supplying lines 18b and 18d transmit a common voltage to the common lines VL1 to VLn.
The common voltage Vcom generated at the common voltage generator 8 is supplied to a plurality of the Ag dots 22 and the first to the fourth common voltage supplying lines 18a to 18d through the first and the second TCP 17a and 17b. Accordingly, the common voltage Vcom is supplied not only to a plurality of common lines VL1 to VLn arranged on the lower substrate 1 but also to a common electrode (not shown) formed on the upper substrate 3. Therefore, the common voltage Vcom is supplied to a plurality of common lines VL1 to VLn for forming the storage capacitor Cst and to the common electrode of the upper substrate 3 through a plurality of the Ag dots 22.
The common voltage Vcom supplied to the common electrode is a driving voltage that drives liquid crystal injected between the upper substrate 3 and the lower substrate 1 with a data voltage supplied to a pixel electrode (not shown). A predetermined image is displayed on the liquid crystal panel 2 by driving the liquid crystal. In order to drive the liquid crystal, a constant common voltage Vcom must be supplied to the common electrode.
Since the liquid crystal panel 2 is of twisted nematic (TN) type and a storage on common structure, a plurality of common lines VL1 to VLn are arranged on the lower substrate 1. As described above, the common voltage Vcom generated at the common voltage generator 8 is supplied to a plurality of common lines VL1 to VLn.
The common voltage Vcom is supplied to the first to the fourth common voltage supplying lines 18a to 18d in an identical manner, a plurality of common lines VL1 to VLn, a plurality of the Ag dots 22 which are formed on the lower substrate 1 and to the common electrode formed on the upper substrate 3.
However, since the common electrode is formed on the entire surface of the upper substrate 3, the common electrode has a greater sheet resistance than a line resistance of the plurality of common lines VL1 to VLn formed on the lower substrate 1. Therefore, the common voltage Vcom generated from the common voltage generator 8 is supplied mostly to the plurality of common lines VL1 to VLn which have comparatively low resistance, and the common voltage Vcom is insufficiently supplied to the common electrode. That is, the common voltage is insufficiently supplied to the common electrode in the related art liquid crystal display device. Such an insufficient supply of the common voltage degrades image quality. Therefore, reliable image quality may not be obtained in the related art liquid crystal display device.